This invention relates to a semiconductor device for detecting and counting gamma rays, and particularly to a semiconductor device for detecting gamma rays wherein the measured count of gamma rays of various energies has improved linearity.
Traditionally, Geiger-Muller counters have been used to detect and quantify gamma rays as well as other types of ionizing radiation. However, the life of these counters is too short, and the linearity of the gamma count with respect to the dose rate is not sufficient. Moreover, a high voltage power supply is needed in such a device.
Therefore, a semiconductor device for detecting gamma radiation has been recently developed. In such a semiconductor device for detecting gamma radiation, a high resistivity material is used, for example, germanium (Ge) or silicon (Si) diffused with lithium (Li). This allows the formation of a depletion layer with respect to gamma rays in the semiconductor material. When gamma rays pass through this depletion layer, secondary electrons are generated by either photoelectric effect, Compton effect, or as a result of the creation of a electron-hole pairs. These secondary electrons then act on grid atoms, and create the electron-hole pairs which are detected as electric pulses. The dosage of gamma rays can be measured by way of counting the number of the electric pulses.
The original function of gamma ray counters was to count the number of gamma rays. The conventional semiconductor device for detecting the radiation suffers from a problem, however, because the number of pulses produced varies according the level of energy of the gamma rays even in the same dose field. The reason for this can be understood from a theoretical consideration of the known device.
The principle of the operation of the known semiconductor device for detecting the radiation is shown in FIG. 1. In the device, an N-type layer 2 is provided in a P-type silicon substrate 1, for example by a diffusion method, to form a PN junction. A reverse bias voltage V.sub.B is applied to the PN junction through electrodes 3 and 4 disposed on two sides of the P-type silicon substrate 1, whereby a depletion layer 5 is formed within the substrate adjacent to the N-type layer. Secondary electrons 7 are formed by one of three processes, a photoelectric effect A, Compton effect B, or creation of electron-hole pairs C when gamma rays 6 which are incident upon the device pass through the depletion layer 5. The secondary electrons 7 then act on grid atoms, as a result of which electron-hole pairs 8, which are detected as electric pulses 9, are created. The number of these electric pulses 9 is counted by a counter 10 having an amplifier. A portion of the gamma rays are not counted, for example as a result of scattering or penetration (gamma ray 11) through the depletion layer 5.
In this device, the number of pulses per unit dosage, namely the count C of the gamma rays, is expressed by the following formula (1). ##EQU1## where: K: constant
.mu..sub.Si : absorption coefficient of radiation detecting device PA1 .mu..sub.air : absorption coefficient of air PA1 l: thickness of depletion layer PA1 S: area of depletion layer PA1 E: energy of gamma rays
As is evident from the formula (1), the value of C is inversely dependent upon E when the thickness 1 of the depletion layer 5, and the area S of the depletion layer 5 along the surface of the silicon substrate 1 are constant. That is, the count C becomes small when the energy of the gamma rays is large. This leads to a reduction in sensitivity for high energy gamma rays, and a deterioration in the quality of radiation measurement.
A semiconductor device for detecting radiation which overcomes the aforesaid problem is shown in Japanese Patent Laid-Open No. 74375/1986, wherein the linearity of the response to variable energy radiation is somewhat improved. The radiation detecting device disclosed in this publication its characterized in that the semiconductor material extends beyond the depletion layer, such that a portion of the gamma rays are incident on this non-depletion region. Secondary electrons which are generated by gamma rays incident upon non-depletion region surrounding the depletion layer, may migrate to the depletion layer and contribute to the signal produced if the mean range of the electron is sufficient to reach the depletion layer from the site of formation.